Technical Abstract:
Disturbances can cause patchiness in the vegetation at different spatial and temporal scales. In arid environments, dominant species of the vegetation are similar to disturbances in that they create patches with a modified biotic and abiotic environment. Shrub invasion in arid environments has caused a shift in the dominant life form, which is expected to have important implications for the occurrence of subdominant plants. The objective of this study was to understand and predict changes in the relative abundance of subdominant species when an evergreen shrub (Larrea tridentata) invades desert grasslands dominated by Bouteloua eriopoda, and to compare those changes to other disturbances that affect the vegetation. We used a spatially-explicit individual-plant based model (ECOTONE) to simulate responses of desert grassland species to shrub invasion using a hierarchical patch dynamics approach. The effects of changes in light, water, and nutrients caused either by disturbance or shrub invasion on subdominant species were evaluated. We found that the effects of shrub invasion on species composition were comparable to other disturbances. However, contrary to other disturbances, changes in processes induced by shrub invasion appear to be self-reinforcing, whereas species recover after disturbances. As a consequence, initially positive effects of shading by shrubs on the abundance of subdominant species ultimately caused a loss of subdominant plants as shrub cover increased. These results correspond to field data collected at a desert grassland-shrubland ecotone at the Sevilleta LTER in New Mexico. Our study demonstrates the similarity and differences of invasive species to disturbances. It also shows how the application of a hierarchical patch dynamics approach can enhance our